(1) Field of the Invention
The present invention relates to a DC-DC converter of a charge pump type which utilizes capacitors. A charge pump type DC-DC converter is widely used in portable electronic devices, such as a cordless telephone set. In order to design the portable electronic device to have various functions and compact dimensions it is necessary to use a stable DC-DC converter which has small voltage variation. A charge pump type DC-DC converter satisfies this requirement.
(2) Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional charge pump type DC-DC converter, which comprises an input terminal 1, switches 2-5, capacitors 6 and 7, and an output terminal 8. Each of the switches 2-5 includes a CMOS (Complementary Metal Oxide Semiconductor). As shown in FIG. 2A, a DC power source, such as a battery 9 generating a voltage V.sub.B, is connected between the input terminal 1 and the ground. A first pair of switches 2 and 3 and a second pair of switches 4 and 5 are alternately turned ON and OFF, so that a desired DC voltage is generated between the output terminal 8 and the ground.
In a state shown in FIG. 2B, the switches 2 and 3 are ON, and the switches 4 and 5 are OFF. A current flows in a direction indicated by solid line X, and thus the capacitor 6 is charged by the battery 9. Next, as shown in FIG. 2C, the switches 2 and 3 are turned OFF, and the switches 4 and 5 are turned ON. In this state a current flows in a direction indicated by solid line Y, so that the capacitor 6 is discharged and the capacitor 7 is charged by the charge stored in the capacitor 6. The above-mentioned switching operation is carried out repeatedly, and thus an output voltage Vo gradually decreases from zero volts and is stably maintained at a voltage -V.sub.B which is the opposite version of the voltage V.sub.B of the battery 9. FIG. 2D shows the relationship between the absolute value .vertline.Vo.vertline. and time t.
FIG. 3 is a circuit diagram of an essential part of a portable electronic device incorporating the DC-DC converter shown in FIG. 1. A predetermined load 10 is connected to the output terminal 8 of the DC-DC converter. The battery 9 is connected to an electronic circuit 11 other than the DC-DC converter. Since the battery 9 has a large internal resistance, a change in current passing through the electronic circuit 11 changes the voltage V.sub.B generated by the battery 9. Thus, the output voltage Vo varies in response to a change in the voltage V.sub.B. Further, the output voltage Vo varies (decreases) due to long-term use of the battery 9 because the electromotive force of the battery 9 decreases. Furthermore, the output voltage Vo varies due to the status of the load 10.
FIG. 4 is a graph of the output voltage Vo as a function of time. The capacitor 7 is charged by the discharging of the capacitor 6 when the switches 2 and 3 are turned OFF and the switches 4 and 5 are turned ON. If the capacitors 6 and 7 have identical capacitances, the output voltage Vo decreases to -V.sub.B ' [V] (a.fwdarw.b). Then, current is supplied to the load 10 by the discharging the capacitors 6 and 7, and hence the output voltage Vo increases (b.fwdarw.c). When the switches 2 and 3 are turned ON and the switches 4 and 5 are turned OFF, current is supplied to the load 10 by the discharging of only the capacitor 7. Hence, the output voltage Vo increases in such a way that an increasing curve obtained during this time (c.fwdarw.d) has a slope approximately twice that obtained between times b and c. As described above, the output voltage Vo of the conventional DC-DC converter shown in FIG. 1 is greatly affected by the load 10.
In order to design the DC-DC converter shown in FIG. 1 so that it operates at a high speed, it is necessary to design the MOS transistors forming the switches 2-5 so that they have small ON resistances. In order to obtain small ON resistances of the MOS transistors, it is necessary to design the MOS transistors so that they have wide areas or design the capacitors 6 and 7 so that they have small capacitances. However, the increasing of the area of each MOS transistor prevents improvement in the integration density, and the decreasing of the capacitance of each of the capacitors 6 and 7 decreases the load driving ability of the DC-DC converter.
FIG. 5 is a circuit diagram of a variation of the conventional DC-DC converter shown in FIG. 1. The output voltage Vo obtained at the output terminal 8 shown in FIG. 5 has a positive voltage higher than the ground potential, while the output voltage Vo obtained at the output terminal 8 shown in FIG. 1 has a negative voltage lower than the ground potential. The circuit shown in FIG. 5 has the same disadvantages as the circuit shown in FIG. 1.